Cytochrome P450s (P450) are a family of enzymes involved in the oxidative metabolism of both endogenous and exogenous compounds. P450 enzymes are widely distributed in the liver, intestines and other tissues (Krishna et al., Clinical Pharmacokinetics. 26:144-160, 1994). P450 enzymes catalyze the phase I reaction of drug metabolism, to generate metabolites for excretion. The classification of P450s is based on homology of the amino acid sequence (Slaughter et al The Annals of Pharmacotherapy 29:619-624, 1995). In mammals, there is over 55% homology of the amino acid sequence of CYP450 subfamilies. The differences in amino acid sequence constitute the basis for a classification of the superfamily of cytochrome P450 enzymes into families, subfamilies and isozymes.
Cytochrome P450 contains an iron cation and is a membrane bound enzyme that can carry out electron transfer and energy transfer. Cytochrome P450, when bound to carbon monoxide (CO), displays a maximum absorbance (peak) at 450 nm in the visible spectra, and is therefore called P450 (Omura et al., J. Biol. Chem. 239:2370, 1964).
Over 200 genes encoding cytochrome P450s have been identified, and are divided among over 30 gene families. These gene families are organized into subfamilies, which vary in regulation of gene expression and in amino acid sequence homology, substrate specificity, catalytic activity, and physiological role of the encoded enzymes. Representative P450 genes and substrates of the encoded enzymes are discussed below.
Listed below are examples of known substrates of members of various P450 subfamilies. See also the discussion in Klassen, ed., Casarett and Doull's Toxicology: The Basic Science of Poisons, McGraw-Hill, 1996, pp. 150 ff. Further information about cytochrome P450 substrates, can be found in Gonzales and other review articles cited above. Current information sources available via the Internet include the “Cytochrome P450 Homepage”, maintained by David Nelson, the “Cytochrome P450 Database”, provided by the Institute of Biomedical Chemistry & Center for Molecular Design, and the “Directory of P450-containing Systems”, provided by Kirill N. Degtyarenko and Peter Fabian.
CYP1A1: diethylstilbestrol, 2- and 4-hydroxyestradiol
CYP1A2: acetaminophen, phenacetin, acetanilide (analgesics), caffeine, clozapine (sedative), cyclobenzaprine (muscle relaxant), estradiol, imipramine (antidepressant), mexillitene (antiarrhythmic), naproxen (analgesic), riluzole, tacrine, theophylline (cardiac stimulant, bronchodilator, smooth muscle relaxant), warfarin.
CYP2A6: coumarin, butadiene, nicotine
CYP2A13: nicotine
CYP2B1: phenobarbital, hexobarbital
CYP2C9: NSAIDs such as diclofenac, ibuprofen, and piroxicam; oral hypoglycemic agents such as tolbutamide and glipizide; angiotensin-2 blockers such as irbesartan, losartan, and valsartan; naproxen (analgesic); phenyloin (anticonvulsant, antiepileptic); sulfamethoxazole, tamoxifen (antineoplastic); torsemide; warfarin, flurbiprofen
CYP2C19: hexobarbital, mephobarbital, imipramine, clomipramine, citalopram, cycloguanil, the anti-epileptics phenyloin and diazepam, S-mephenyloin, diphenylhydantoin, lansoprazole, pantoprazole, omeprazole, pentamidine, propranolol, cyclophosphamide, progesterone
CYP2D6: antidepressants (imipramine, clomipramine, desimpramine), antipsychotics (haloperidol, perphenazine, risperidone, thioridazine), beta blockers (carvedilol, S-metoprolol, propafenone, timolol), amphetamine, codeine, dextromethorphan, fluoxetine, S-mexiletine, phenacetin, propranolol
CYP2E1: acetaminophen; chlorzoxazone (muscle relaxant), ethanol; caffeine, theophylline; dapsone, general anesthetics such as enflurane, halothane, and methoxyflurane; nitrosamines
CYP3A4: HIV Protease Inhibitors such as indinavir, ritonavir, lopinavir, amprenavir, tipranavir, darunavir, and saquinavir; HIV integrase inhibitors such as raltegravir, Hepatitis C virus (HCV) protease inhibitors, benzodiazepines such as alprazolam, diazepam, midazolam, and triazolam; immune modulators such as cyclosporine; antihistamines such as astemizole and chlorpheniramine; HMG CoA Reductase inhibitors such as atorvastatin, cerivastatin, lovastatin, and simvastatin; channel blockers such as diltiazem, felodipine, nifedipine, nisoldipine, nitrendipine, and verapamil; antibiotics such as clarithromycin, erythromycin, and rapamycin; various steroids including cortisol, testosterone, progesterone, estradiol, ethinylestradiol, hydrocortisone, prednisone, and prednisolone; acetaminophen, aldrin, alfentanil, amiodarone, astemizole, benzphetamine, budesonide, carbamazepine, cyclophosphamide, ifosfamide, dapsone, digitoxin, quinidine (anti-arrhythmic), etoposide, flutamide, imipramine, lansoprazole, lidocaine, losartan, omeprazole, retinoic acid, FK506 (tacrolimus), tamoxifen, taxol and taxol analogs such as taxotere, teniposide, terfenadine, buspirone, haloperidol (antipsychotic), methadone, sildenafil, trazodone, theophylline, toremifene, troleandomycin, warfarin, zatosetron, zonisamide.
CYP6A1: fatty acids.
The efficacy of a drug can be dramatically affected by its metabolism in the body. For drugs that are rapidly metabolized it can be difficult to maintain an effective therapeutic dose in the body, and the drug often must be given more frequently, in higher dose, and/or be administered in a sustained release formulation. Moreover, in the case of compounds for treating infectious disease, such as viral or bacterial infections, the inability to maintain an effective therapeutic dose can lead to the infectious agent becoming drug resistant. Many compounds that have strong biological efficacy and that would otherwise be potentially powerful therapeutics are rendered essentially useless by virtue of their short half-lives in vivo. A common pathway of metabolism for drugs containing lipophilic moieties is via oxidation by one or more cytochrome P450 enzymes. These enzymes metabolize a drug to a more polar derivative that is more readily excreted through the kidney or liver. First pass metabolism refers to the elimination of drugs via liver and intestinal CYP450 enzymes. First pass metabolism can lead to poor drug absorption from the GI tract due to extensive intestinal CYP450 metabolism, low plasma blood levels due to hepatic CYP450 metabolism, or both. Poor oral bioavailability due to CYP450 metabolism is a major reason for the failure of drugs candidates in clinical trials. In some instances, metabolic by-products of CYP450 enzymes are highly toxic and can result in severe side effects, cancer, and even death.
Some examples of the effects of drug metabolism by CYPs include:
Acetaminophen: Ethanol up-regulates CYP2E1, which metabolizes acetaminophen to a reactive quinone. This reactive quinone intermediate, when produced in sufficient amounts, causes liver damage and necrosis.
Sedatives: The sedative phenobarbital (PB) up-regulates several P450 genes, including those of the CYP2B and CYP3A subfamilies. Upregulation of these enzymes increases the metabolism and reduces the sedative effects of PB and the related sedative hexobarbital.
Antibiotics: The antibiotics rifampicin, rifampin, rifabutin, erythromycin, and related compounds are inducers of the CYP3A4 gene and are substrates of the enzyme product.
Anti-cancer agents: Taxol and taxotere are potent anti-cancer agents. Both drugs are extensively metabolized by CYP3A4 and have poor oral bioavailability. These drugs are only efficacious in parenteral formulations which, due to their poor solubility properties, are highly noxious to patients.
Nicotine: CYP2A6 and 2A13 convert nicotine, a non-toxic component of cigarette smoke, into NNK, a highly potent carcinogen that contributes to lung cancer from smoking.
Oral contraceptive/estrogen replacement therapy: Estrogens and estradiols are the active ingredients in oral contraceptives and in hormonal replacement therapies for post-menopausal women. Women who are also taking antibiotics such as rifampicin or erythromycin, or glucocorticoids such as dexamethasone, or who smoke, risk decreased efficacy of the estrogen/estradiol treatments due to increased metabolism of these compounds by up-regulated CYP3A4 and/or CYP1A2 enzymes.
Dextromethorphan: CYP2D6 metabolizes dextromethrophan to dextrorphan. Individuals who express high levels of CYP2D6 (so-called rapid metabolizers) do not receive therapeutic benefits from dextromethorphan due to extensive first-pass metabolism and rapid systemic clearance.
Protease Inhibitors: Protease inhibitors and non-nucleoside reverse transcriptase inhibitors currently indicated for use in treatment of HIV or HCV are typically good substrates of cytochrome P450 enzymes; in particular, they are metabolized by CYP3A4 enzymes (see e.g. Sahai, AIDS10 Suppl 1:S21-5, 1996) with possible participation by CYP2D6 enzymes (Kumar et al., J. Pharmacol. Exp. Ther. 277(1):423-31, 1996). Although protease inhibitors are reported to be inhibitors of CYP3A4, some non-nucleoside reverse transcriptase inhibitors, such as nevirapine and efavirenz, are inducers of CYP3A4 (see e.g. Murphy et al., Expert Opin Invest Drugs 5/9: 1183-99, 1996).
Human CYP450 isozymes are widely distributed among tissues and organs (Zhang et al., Drug Metabolism and Disposition. 27:804-809, 1999). With the exception of CYP1A 1 and CYP2A13, most human CYP450 isozymes are located in the liver, but are expressed at different levels (Waziers J. Pharmacol. Exp. Ther. 253: 387, 1990). A solution to the problem of drug degradation and first-pass metabolism is to control the rate of drug metabolism. When the rates of drug absorption and metabolism reach a steady state, a maintenance dose can be delivered to achieve a desired drug concentration that is required for drug efficacy. Certain natural products have been shown to increase bioavailability of a drug. For example, the effect of grapefruit juice on drug pharmacokinetics is well known. See Edgar et al., Eur. J. Clin. Pharmacol. 42:313, (1992); Lee et al., Clin. Pharmacol. Ther. 59:62, (1996); Kane et al., Mayo Clinic Proc. 75:933, (2000). This effect of grapefruit juice is due to the presence of natural P450-inhibiting components. Other compounds also have been used for inhibition of P450. For example, the HIV-1 protease inhibitor Ritonavir® is now more commonly prescribed for use in combination with other, more effective, HIV protease inhibitors because of its ability to “boost” those other compounds by inhibiting P450-mediated degradation.
Present methods of inhibiting cytochrome P450 enzymes are not wholly satisfactory because of toxicity issues, high cost, and other factors. For example, using ritonavir to inhibit cytochrome P450 is not desirable in disorders other than HIV infection. It is apparent, therefore, that new and improved methods of inhibiting cytochrome P450 enzymes are greatly to be desired. In particular, methods where an inhibitor can be co-administered with another biologically active compound that is metabolized by cytochrome P450 enzymes are highly desirable.